1. Technical Field
The present invention relates to pumps, and more particularly to a control and a method for controlling the rate at which liquid is pumped through a tube.
2. Background Art
Transfer of a liquid at a precise rate is a requirement of many liquid delivery systems such as, for example, intravenous chemical therapy systems wherein a solution, such as a saline solution or a solution containing medicine, is transferred by a pump from a supply to a patient requiring the solution. Safety and effectiveness of the therapy is affected by the degree of control with which the solution is transferred.
A peristaltic infusion pump typically is used to draw liquid from a container and pump the liquid through the outlet of a flexible tube. The peristaltic pump is connected to the tube between the container and the outlet and has a rotatable roller engaged with the tube exterior for periodically squeezing / collapsing the tube and pumping liquid through the tube.
The rate at which liquid is pumped through the tube by the peristaltic pump is a function of the speed at which the pump shaft rotates (that is, the pump speed) and the cross sectional area of the tube bore. The cross sectional area of a bore in a flexible tube can vary by plus and minus ten percent along the length of the tube. In addition, wear of the inner surface of a tube results in a change in the tube bore cross sectional area. Squeezing of a tube by a peristaltic pump can cause the circumference of the tube to expand whereby the enclosed area of a cross section of the tube increases. Pump speed must be continuously compensate for variations in the cross sectional area of a tube to maintain a desired flow rate of liquid in a tube.